Process for soldering electronic components to a printed circuit board

ABSTRACT

The invention concerns a method for soldering of additional electronic components onto a circuit board having components which have already been soldered and mounted thereto, wherein the additional components are plugged through the circuit board and soldered thereto. In order to be able to subsequently solder the components in a simple and reliable fashion, a template, having a pattern of holes corresponding to the soldering points and locations which are to be subsequently formed on the circuit board, is lowered and pressed onto the surface of a solder bath in such a fashion that the solder displaces into the holes and rises up within same. The circuit board having the previously mounted additional components is placed onto the template in such a fashion that the regions which are to be soldered are disposed within the openings of the template and dipped into the solder located therein. After this soldering process, the circuit board and the template are lifted from the surface of the soldering bath and this surface can be subsequently cleaned.

BACKGROUND OF THE INVENTION

The invention concerns a method for soldering additional electroniccomponents to a circuit board which already has soldered components,wherein the additional components are plugged through the circuit boardand soldered thereto.

In order to solder electronic components to a circuit board,conventional technology has utilized the so-called feed-through mountingtechnique with which thin component connectors, so-called legs, areplugged from one side into holes of the circuit board and connectedthereto at the rear side through the introduction of solder. Towardsthis end, the so-called flow solder method is normally utilized withwhich the liquid solder, in the form of a wave, is introduced to therear side of the circuit board. This method has the disadvantage thatthe circuit board can thereby only be configured at one side.

In addition, the so-called surface mounting technique has been developedwith which the components are disposed on the surface of the circuitboard and soldered without having to pass through the circuit board. Inthis manner, the circuit board can be configured on both sides. In thiscase as well it is possible for the solder to be introduced using theflow solder method however the so-called reflow technique is usuallyutilized. Paste solder having a resin component is thereby used andtherefore simultaneously serves for attaching as well as soldering thecomponents. After introduction of the paste solder, the components areinserted therein and introduced into an oven at the position fixed bythe resin component. The paste solder then melts and connects thecomponents to the circuit board.

It is often necessary to retroactively solder additional electroniccomponents onto a circuit board which already has electronic componentsintroduced thereon in accordance with one of the two methods. Onetherefore has the difficulty, that the subsequent soldering can damagethe elements which are already present on the circuit board as well astheir soldered connections. This is not acceptable. The subsequentlymounted components are therefore plugged through the circuit board inthe above mentioned fashion and, connected to the circuit board usingsingle soldering points and soldering regions.

Known in the art from DE 43 14 241 A1 is the effecting of individualsoldering points or regions using a small soldered head on the upperexit end of which the solder, in the form of a small wave, overflows sothat a so-called mini-wave is formed. It has turned out that this typeof small solder head does not effect reliable soldering results, sincethe solder has a high affinity for oxygen so that impurities in the formof an oxidation layer are often present on the surface of the solderhead. This layer is difficult or impossible to remove. In addition, ithas turned out that the height of the wave has certain fluctuations.This can encroach upon the quality of the solder joint. In addition, therelatively small amount of solder can lead to an uneven solderdistribution.

SUMMARY OF THE INVENTION

The soldering points or soldering regions which are to be subsequentlyeffected are usually unevenly distributed over the circuit board. In themethod mentioned, this leads to the additional disadvantage that thecircuit board must either be displaced above the solder head during thesoldering process, or a plurality of solder heads must be providedcorresponding to the number of soldering points to be formed. In thefirst case, very complicated movements are required, making the methodslow and not very effective and the apparatus is difficult to constructand correspondingly expensive. In the second case, the constructionaleffort is likewise extremely high and there is an unacceptably high riskof poor quality soldering in consequence of impurities in conjunctionwith the large number of soldering heads.

U.S. Pat. No. 5,176,312 proposes a device for the soldering ofelectronic components to a circuit board with which the upper side ofthe soldering bath has an attached nozzle plate having a plurality ofdiffering nozzles which are disposed in correspondence with the pointsor regions which are to be soldered and through which the solder isintroduced thereon using a pump. Since these nozzles have differingcross sectional shapes and diameters, introduction of the solder intothe nozzles results in differing flow and pressure dependencies whichare difficult to control. In addition, the nozzles, due to their narrowdiameters, can easily become soiled in the above mentioned manner orblocked to encroach upon the soldering results.

A similar solution is described in AT 297 827 with which solderconducting sheet metal having holes is disposed on the upper end of awell forming the solder bath. The solder can be raised within the wellusing a transport device so that it rises into the holes to build convexsolder caps on the upper surface thereof. The raising of the solderwithin the well and the holes leads to the above mentioned disadvantageswith regard to difficulty of controlling the flow and pressuredependencies.

Known in the art from U.S. Pat. No. 4,739,919 is the soldering ofadditional electronic components onto a circuit board which already hassoldered components. In order to protect same, a metal template isprovided on which the circuit board is disposed in such a fashion thatthe elements which are already soldered are accepted in recesses of themetal template. The soldering machine passes over the circuit board onthe metal template so that a plurality of metal templates are necessaryfor operation of the soldering machine which is very difficult andexpensive. The additional components are attached to the circuit boardusing a wave soldering method in which a solder wave or a solder flow isintroduced on that side of the metal template facing away from thecircuit board and rise into openings of the metal template. This processalso has the above mentioned disadvantages. In addition, the use of ametal template in the method in accordance with U.S. Pat. No. 4,739,919is particularly difficult, since metals have a relatively largecoefficient of heat expansion. When the solder comes in contact with themetal template, rapid local temperature increases are effected in themetal template as a result of which the metal template deforms so that acorrect alignment of the circuit board is no longer guaranteed.

It is the underlying purpose of the invention to create a method of theabove mentioned kind with which electronic components can beretroactively soldered to a circuit board in a simple and reliablefashion.

This method is achieved in accordance with the invention in that thetemplate, having a pattern of holes corresponding to the solderingpoints or regions which are to be retroactively introduced onto thecircuit plate, is lowered and pressed onto the surface of a solder bathin such a fashion that the solder is displaced and spreads into theopenings and rises therein without overflowing. The circuit board isintroduced into the template having the pre-mounted additionalcomponents in such a fashion that the regions to be soldered aredisposed within the holes in the template and dip into the solderlocated therein, wherein the circuit board and the template aresubsequently raised from the surface of the solder bath.

In accordance with the invention, the solder and the points or regionswhich are to be subsequently soldered are not thereby introduced usingspecial nozzles or a mini-wave, rather a dipping procedure is proposedwith which only those regions or points of the components or the circuitboard are dipped into the solder bath which are to be joined withsolder. Towards this end, the template is lowered onto the surface ofthe solder bath as a result of which those sections of the surface ofthe solder bath which are not intended to come in contact with thecircuit board are covered. The template is thereby used to prepare aplurality of small solder surfaces into which the corresponding sectionsof the circuit board and of the components which are to be soldered canbe dipped. Since the pressure increase in the solder bath is notgenerated by means of a pump, rather is effected simply through thecomplete area sinking of the template onto the surface of the bath, aneven rising of the solder into the holes in the template is achieved.

A first embodiment of the method in accordance with the invention hasturned out to be very useful in which the template is initially loweredonto the surface of the solder bath and pressed thereon as result ofwhich the solder is displaced and rises into the openings so that theplurality of dipping baths are formed. The circuit board is then placedonto the template, wherein the regions to be soldered dip into thesubmersion bath. It is alternatively however possible to initially placethe circuit board on the template and subsequently lower both togetheronto the surface of the solder bath.

In order to exclude risks from impurities due to formation of anoxidation layer on the surface of relatively small solder baths, thetemplate is lifted from the solder bath after the circuit plate ortogether therewith so that the entire surface of the solder bath is oncemore accessible and can be cleaned in a simple fashion. Cleaning can beeffected using a wiping blade or by controlled overflow. In this manner,one guarantees that the small solder baths formed on the surface by thetemplate are also substantially free from impurities.

The method in accordance with the invention facilitates high circuitboard throughput, since the individual soldering points or regions canall be simultaneously soldered without requiring a transverse motion ofthe circuit board. One thereby also has the associated advantage thatthe repetition rate is independent of the number of soldering locations.In addition, the method in accordance with the invention has turned outto be extremely reliable since, due to the material properties of thesolder (density, viscosity, flow dependence), large amounts are easierto control than the small amounts of solder of prior art.

If the circuit board with the previously mounted components is equippedusing the feed-through mounting technique mentioned above, thecomponents are only disposed on that side of the circuit board facingaway from the solder bath. If however the components are disposed onboth sides of the circuit board using surface mounting, it can benecessary to protect those components on the circuit board at the sidethereof facing the solder bath. Towards this end, an improvement of theinvention provides that the template has recesses at its side facingaway from the surface of the solder bath into which the components whichare already mounted to the circuit board can be accepted during thesoldering process. These recesses, in addition to providing protectionwith respect to the solder, have the additional advantage that thecircuit board can be lowered deeper into the template as a result ofwhich one guarantees that the points or regions to be soldered reliablydip into the small solder baths. In this fashion, in accordance with theinvention, a relatively large dipping depth is possible, since thetemplate also facilities covering of very tall components or mechanicalelements on the circuit board using corresponding recesses or integralcovering caps.

In order to facilitate the reliable and precise motion of the templatebetween the lowered and raised positions, an improvement of theinvention provides that the template be driven along a guide between araised position separated from the surface of the solder bath and alowered position pressed against the surface of the solder bath. Theguide can thereby comprise at least two guide pins which are mounted toa housing and which engage with close tolerance in corresponding throughholes of the template.

In order to effect very precise solder connections, the circuit boardshould be guided and positioned relative to the template by means of aguide device. The circuit board is thereby preferentially disposed on aholding frame which is guided relative to the template and can belowered and raised relative thereto. The guide pins guiding the templaterelative to the solder bath, can also be used to guide the holding framerelative to the template.

In order to be able to utilize the solder bath for differing componentconfigurations, the template is preferentially exchangeable. Thisfacilitates a rapid and constructively simple adjustment to differingpatterns of solder points to be formed. The exchangeable template hasthe additional advantage that the heavy, hot and expensive solder bathmust not be reconfigured, since adjustment to different configurationsis effected only by the exchange of the template.

When soldering electronic components, one normally introduces flux ontothe surface to be covered prior to the actual soldering procedure toreduce or eliminate the surface oxidation for achieving a bettersoldered connection. When elements are retroactively soldered, the fluxshould be introduced only at those points or regions of the circuitboard which are to be soldered. This can be done in a simple fashion inaccordance with the invention in that the flux is sprayed with theintermediate insertion of a spray template likewise having a pattern ofholes corresponding to the soldering points or regions which are to beretroactively introduced onto the circuit board. One can therebyguarantee that the flux is only introduced onto the circuit board at thedesired locations. The undesirable over spray which normally condenseson the circuit board is blocked by the spray template.

A preferred embodiment provides that the circuit board be positionedabove the spray template and that the flux be sprayed from below. Ifappropriate, the circuit board can then be pre-warmed in a conventionalfashion.

Further details and features of the invention can be extracted from thesubsequent description of the embodiments with reference to the drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1a, 1b, 1c, 1d, 1e and 1f show the individual phases of a firstembodiment of the method in accordance with the invention,

FIGS. 2a, 2b, 2c, 2d, 2e and 2f show the individual phases of a secondconfiguration of the method in accordance with the invention, and

FIG. 3 shows a schematic representation of an installation for carryingout the method.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The individual phases of a first embodiment of the method in accordancewith the invention are shown in FIGS. 1a through 1f.

Liquid solder 6 is located in an outer vessel 1 in which an inner vessel5 is inserted into which the solder can flow via inlet openings (notshown). The solder bath 7 disposed in the inner vessel 5 can be held ata predetermined defined pressure by means of a pump comprising a pumpwheel 3 driven by an electrical motor 4 via a drive shaft 2, so that thesurface of the solder bath maintains a predetermined desired level.

A circuit board 10, of conventional construction, is already equippedwith pre-mounted and soldered components 13, so-called SMDs, using thesurface mounting method. The method in accordance with the invention isthen used to solder additional electronic components 12 to the circuitboard 10 which are plugged through the circuit board 10 in a previousprocessing step (not shown), as illustrated in FIG. 1a. The circuitboard 10 is placed on a holding frame 9 and is passed through differingstations of a soldering machine.

A template 8, having a plurality of openings 11, is disposed over thesurface 7a of a solder bath 7 located in the inner vessel 5. Thepositions of the openings 11 correspond to the soldering points orregions which are to be subsequently formed on the circuit board 10 tosolder components 12. The template 8 also has a plurality of recesses 8aon its surface facing away from the solder bath 7 and towards thecircuit board 10 into which the components 13 which have already beensoldered to the circuit board can be introduced during the solderingprocess. As indicated in FIG. 1a by means of the arrow P₁, the holdingframe 9 having the circuit board 10 is initially disposed above thetemplate 8.

In a subsequent processing step, the template 8 is lowered out of theposition shown in FIG. 1a, in which it is disposed above the surface 7aof the solder bath 7, into the surface 7a of the solder bath (arrow P₂)until the surface 7a of the solder bath 7 is covered and the solder inthe bath 7 is pressed through the openings 11 in the template 8 to risetherein, without however overflowing (FIG. 1b). In this manner, aplurality of small solder baths 7b are formed whose relative positionscorrespond to the pattern of soldering points and regions to be formedon the circuit board 10. The template 8 is guided by vertical guidingpins 14 during the lowering process which are introduced on the innervessel 5 and engage into corresponding through holes 15 of the template8 under close tolerance.

The holding frame 9 having the circuit boards 10 is subsequently loweredonto the template 8 (arrow P₃), wherein the free ends of the guidingpins 14 penetrating through the template 8 engage into correspondingbore holes 16 of the holding frame 9 such that the holding frame 9, withthe circuit board 10 located thereon, is guided relative to the template8 and relative to the solder bath 7. As shown in FIG. 1c, the components13 which have already been soldered, thereby gain entrance into therecesses 8a of the template 8 and are protected in this manner. Thecontacts or legs of the components 12 which are to be soldered protrudein a downward direction and dip into the small solder baths 7b formed inthe openings 11 of the template 8. The holding frame 9 remains in thislocation along with the circuit board 10 for about 2 to 3 seconds.

In a subsequent method step, the holding frame 9 with the circuit board10 (arrow P₄, FIG. 1d) and, subsequently, the template 8 are lifted fromthe solder bath 7 (arrow P₅ in FIG. 1e). The holding frame 9 with thecircuit board 10 is then transported within the machine to the nextprocessing station (arrow P₆). In a final method step, the freed surfaceof the solder bath is cleaned by means of a wiping blade 17 to removepossible oxidation remnants (FIG. 1f), wherein the impurities areexpelled into a overflow vessel 18.

Individual phases of a second embodiment of the method in accordancewith the invention are shown in FIGS. 2a through 2f, wherein these stepsdiffer from the method in accordance with FIGS. 1a through 1f solely inthat the holding frame 9 having the circuit board 10 is initiallylowered onto the template 8 (FIG. 2b) before these components aremutually dipped onto the surface of the solder bath (FIG. 2c). After thesoldering locations have formed, the template 8 together with theholding frame 9 and the circuit board 10 are lifted from the solder bath7 (FIG. 2d) and, subsequent thereto, the holding frame 9 is lifted fromthe template 8 and led to an additional processing station. Alladditional processing steps have already been described in theembodiment of the method in accordance with FIGS. 1a through 1f, towhich reference is hereby made.

FIG. 3 shows a schematic view of a soldering installation in which theindividual stations are shown. In the left station I in accordance withFIG. 3, the circuit boards are first manually or automatically placedonto the holding frames. They then pass, together with the holdingframes, through the soldering installation in a counterclockwisedirection, wherein flux is introduced in station II. Since the flux canonly be introduced into those regions which are to be subsequentlysoldered, in accordance with the invention, the flux is sprayed-on withintermediate disposition of a spray template, wherein the spray templatelikewise has a pattern of holes corresponding to the soldering points orregions which are to be subsequently introduced onto the circuit board.

In the event that additional preparation for soldering is provided for,this can be carried out in the next station III. In the installationshown in FIG. 3, the stations IV, V and VI serve for pre-heating of thecircuit board, whereas the actual soldering process transpires instation VII.

The stations VIII, IX and X facilitate a cooling of the circuit boardfollowing soldering, subsequent to which the completed soldered circuitboard is once more removed from the holding frame at station I.

We claim:
 1. Method for soldering additional electronic components ontoa circuit board having previously soldered components, with a templatehaving a pattern of openings corresponding to soldering points andregions which are to be soldered onto the circuit board, with a solderbath, and with means for guiding the template between a raised positionabove a surface of the solder bath and a lowered position pressingagainst the surface of the solder bath, the method comprising the stepsof:a) plugging the additional components through the circuit board; b)lowering the template along the guiding means to press onto the surfaceof the solder bath, wherein solder rises into the openings c) placingthe circuit board having the previously soldered components, onto thetemplate following step b), wherein the regions which are to be solderedare disposed within the openings in the template and dip into thesolder; and d) lifting the circuit board and the template from thesurface of the solder.
 2. The method of claim 1, wherein the guidecomprises at least two guide pins.
 3. The method of claim 1, furthercomprising means for positioning the circuit board relative to thetemplate.
 4. The method of claim 1, further comprising disposing thecircuit board on a holding frame and guiding the holding frame relativeto the template.
 5. The method of claim 4, wherein the guide comprisesat least two guide pins, the guide pins guiding the holding framerelative to the template.
 6. The method of claim 1 further comprisingthe step of exchanging the template for another template.
 7. The methodof claim 1, further comprising the step of cleaning the surface of thesolder bath following step d).
 8. The method of claim 1, wherein thetemplate has, on a side thereof facing away from the surface of thesolder bath, recesses in which the previously soldered components areaccepted.
 9. The method of claim 1, further comprising the steps ofpositioning a spray template having a pattern of holes corresponding tothe soldering points and the regions which are to be subsequently formedon the circuit board and spraying flux through the template holes ontothe points and regions of the board which are to be soldered.
 10. Themethod of claim 9, wherein the circuit board is positioned above thespray template and the flux is sprayed-on from below.
 11. The method ofclaim 1, further comprising the step of pre-warming the circuit board.12. Method for soldering additional electronic components onto a circuitboard having previously soldered components, with a template having aplurality of openings corresponding to soldering points in regions whichare to be soldered onto the circuit board, with a solder bath and withmeans for guiding the template between a raised position above thesurface of the solder bath and a lowered position pressing against thesurface of the solder bath, the method comprising the steps of:a)plugging the additional components through the circuit board; b) placingthe circuit board, having the previously soldered components onto thetemplate, wherein the regions which are to be soldered are disposedwithin the openings of the template; c) lowering the template along theguiding means following step b) to press onto the surface of the solderbath, wherein solder rises into the openings and the regions to besoldered dip into the solder; and d) lifting the circuit board and thetemplate from the surface of the solder.
 13. The method of claim 12,wherein the guide comprises at least two guide pins.
 14. The method ofclaim 12, further comprising means for positioning the circuit boardrelative to the template.
 15. The method of claim 12, further comprisingdisposing the circuit board on a holding frame and guiding the holdingframe relative to the template.
 16. The method of claim 12, wherein theguide comprises at least two guide pins, the guide pins guiding theholding frame relative to the template.
 17. The method of claim 12,further comprising the step of exchanging the template for anothertemplate.
 18. The method of claim 12, further comprising the step ofcleaning the surface of the solder bath following step d).
 19. Themethod of claim 12, wherein the template has, on a side thereof facingaway from the surface of the solder bath, recesses in which thepreviously soldered components are accepted.
 20. The method of claim 12,further comprising the step of positioning a spray template having apattern of holes corresponding to the soldering points and the regionswhich are to be subsequently formed on the circuit board and sprayingflux through the template holes onto the points and regions of the boardwhich are to be soldered.
 21. The method of claim 12, wherein thecircuit board is positioned above the spray template and the flux issprayed on from below.
 22. The method of claim 12, further comprisingthe step of pre-warming the circuit board.